In agricultural machines, such as, for example, telescopic loaders, wheel loaders, or front loaders on tractors, it is known practice to apply a hydraulic spring support system that provides spring support for the boom or the linkage in order to attain an improved spring suspension comfort, particularly during the operation. Here the lifting side of the hydraulic cylinder is connected to a hydraulic accumulator by means of an appropriate hydraulic arrangement of valves, in order to provide spring support by the hydraulic accumulator. Furthermore the lowering side of the hydraulic cylinder is connected to a hydraulic reservoir, in order, on the one hand, to avoid cavitation during the lowering and, on the other hand, to permit free movement of the piston rod during the spring support process. To improve safety against a sudden sinking of the boom or the linkage, these spring support systems can be equipped with load holding valves to secure these systems against hose breakage. However it is then necessary, in order to permit a lowering of the hydraulic cylinder, to close the reservoir connection on the lowering side of the cylinder, so that a sufficient pressure is built up in order to open the load holding valve. Only after opening the load holding valve can hydraulic fluid drain off from the lifting side of the hydraulic cylinder.
A hydraulic arrangement for such a spring support system is disclosed in EP 1 157 963 A2. A spring support system is proposed for the boom of a telescopic loader that is provided with a load holding valve or an automatic shut-off valve in order to secure the boom against falling off. A separate selector valve is arranged in order to be able to open the load holding valve on the one hand, and on the other hand, to make available a spring support function even in the neutral position of the hydraulic cylinder. The valve must be closed so as to close a connection to the reservoir established for the spring support in order to be able to build up the pressure in the supply line needed to open the load holding valve. This condition makes it necessary for the “lowering” function of the hydraulic cylinder to be detected or monitored at an appropriate location and must be considered in the switching logic of the spring support arrangement for the closing of the control valve, which has been found to be particularly costly and problematical in the case of purely mechanically actuated controllers. In this connection EP 1 157 963 A2 points to a monitoring arrangement in the form of a sensor on the controller that is to determine whether or not the boom is to be lowered. Without a fixed monitoring arrangement or with a defective monitoring arrangement for the controller or for the “lowering function” switching errors could occur in the hydraulic arrangement.